C-clamps with a long bar and two clamp heads have been used for many years. The C-clamps have at least one clamp head that is moveable to a plurality of positions along the length of a bar and a fixed clamp head in some C-clamp designs. Both clamp heads are moveable along the length of a long bar in other C-clamp designs.
The majority of the C-clamps with a long bar have employed mechanical locks to lock clamp heads relative to the long bar. The mechanical locks generally require a special, dedicated long bar. These special, dedicated long bars may have slots, apertures, grooves, or other structures to position a clamping head. Dedicated long bar members are relatively expensive and are difficult to transport and store, due to their length.
A few C-clamps with bars and clamp heads have special clamps or wedge members for holding a clamp head in a selected position along the length of a long bar. Special clamps for holding a clamp head are expensive, subject to failures and frequently permit slippage. Some wedge-type retainers have moving parts and may even have springs. The moving parts are prone to breakage. The wedges are also prone to locking up and being very difficult to release.
A few clamp heads used with long bars to produce a C-clamp with an adjustable throat produce high stress concentrations between clamp heads and long bars. Such stress concentrations can damage long bars. A damaged long bar must be replaced at substantial expense. Some clamp heads with high stress concentrations are used only with special, long bars made from hardened steel. Such long bars are expensive and may allow clamp heads to slip.